Natal Philopatry and Close Inbreeding in Cory's Shearwater (Calonectris diomedea)

Close inbreeding is known in several bird species, although it is seldom observed (Greenwood et al. 1978, Craig and Jamieson 1988, Gibbs and Grant 1989, Bensch et al. 1994). Inbreeding can have important evolutionary consequences because it may carry both costs and benefits to fitness (Mayr 1963, Shields 1983, Greenwood 1989). However, the seemingly low rate of inbreeding in nature may result from the fact that documenting inbreeding requires long-term monitoring of marked individuals. Colonial waterbirds are good candidates for studies of inbreeding because they often exhibit high natal philopatry (Greenwood and Harvey 1982). Moreover, long-term studies allowing individual identification and monitoring are relatively easy and widespread in this group (Wooller et al. 1992). Curiously, however, only two cases of inbreeding have been reported for colonial waterbirds: Yellow-eyed Penguin (Megadyptes antipodes; Richdale 1957), and Mute Swan (Cygnus olor; Coleman et al. 1994). Here, we report two cases of close inbreeding in Cory's Shearwater (Calonectris diomedea) documented during an 18-year study. We also consider philopatric behavior in this species and its possible relationships with inbreeding. Cory's Shearwaters are highly philopatric (Ristow et al. 1990, Thibault 1993, Borg and Cachia-Zammit 1997), breeding only on islands in the Mediterranean Sea (C. d. diomedea), and along the west coast of Africa from the Cape Verde islands (C. d. edwardsii) to the Macaronesian and Berlenga islands (C. d. borealis). The biology of this species has been well documented, and geographic variation in biometric (Massa and Lo Valvo 1986), vocal (Bretagnolle and Lequette 1990), and genetic (Randi et al. 1989, Wink et al. 1993) traits tend to confirm this extreme philopatric behavior. Methods.-Our study was conducted on the Lavezzi Islands, south Corsica (300 to 400 breeding pairs). All adults and fledglings were banded and recaptured annually between 1978 and 1996 in five subcolonies totaling 104 to 133 nests (Thibault 1993); 1,045 chicks were banded during this period. Prospecting birds also have been recaptured, albeit irregularly (but more thoroughly since 1992; see Thibault 1993). Recaptured birds were sexed using bill dimensions that have about 95% predictive power (Ristow and Wink 1980, Granadeiro 1993, Thibault 1993).